Help needed Multi city flight search logic

I would like to create flight booking system in CI4. I know the search logic for single and return way, but I couldn't figure out the logic for Multi city flight search like multi city search in expedia.com. 

Guys,
I really appreciate, if anybody could help me to achieve multi city logic in PHP. 

Guys Help please

If every option/input/dropdown etc on the search area is selected, I believe it will be the same with the "one input search" you can already do. 
Just send the values to controller, then to model and retrieve the results by filtering according to the values.

However, if there are some options which might be blank, you can check this answer written for one of my questions : 

https://forum.codeigniter.com/thread-74617.html

Good Luck.

I would like to create flight booking system in CI4. I know the search logic for single and return way, but I couldn't figure out the logic for Multi city flight search like multi city search in expedia.com. 

Guys,
I really appreciate, if anybody could help me to achieve multi city logic in PHP. 

well you can send it as an ARRAY of selected cities to the controller

Thanks for hints guys

I may also require another help.

Is there any free flight search API available (atleast with limitations)?